Monday, June 17

Mirages and sandstorms


A massive sandstorm rolls into Yazd, Iran in April 2018. – Reuters file photo

WE often associate these two atmospheric phenomena with hot desert landscapes but in all truth they can be experienced in most parts of the globe. A mirage is an optical illusion which we wonder at or admire. The English word ‘mirror’ is derived from the linguistic root. Sandstorms can also occur on beaches when onshore winds whip up dry sand and deposit it on coastal dunes which, in time, migrate inland.

Inferior mirages

Most of us have witnessed these whilst driving around Kuching on a hot, dry day, and even here in England under summer anticyclonic weather with air of a low relative humidity. We perceive ’pools of water’ on asphalt roads when nearing the crest of a small hill. On many an unfamiliar road, I have instinctively reduced the car’s speed just in case these pools are deep, only to find in reality water wasn’t present! Such an illusion can be seen at airports where jet engine exhausts on asphalt runways cause a heat-haze, shimmering effect. The heat of the overhead sun is absorbed by the tarmac surfaces, causing a superheated layer of air at ground level to rise up to a metre where that layer of air meets with slightly denser colder air. In so doing, it creates a gradient in the refractive index of the air. Thus, sunlight is refracted and the pool of water is no more than a reflection of the sky above.

Both asphalt and sand surfaces, when exposed to strong sunlight in air of less than 5 per cent relative humidity, are heated up and hotter than the air above by 10 degrees Celsius. Such mirages are commonly referred to as ‘road images’ or ‘desert images’. Meteorologists call this blurred, shimmering effect an ‘inferior mirage’, often portrayed in films showing saddle-sore and weary travellers in camel trains crossing the Sahara or Arabian deserts and perceiving an oasis. Their camels know better for they can detect the scent of water in a real oasis.

Superior mirages

In such cases, the air nearest to the ground is cooler than the descending air above it and is referred to as a ‘temperature inversion’. In this mirage, the sun’s rays from above pass through the temperature inversion and are bent downwards so that the image appears above the true object at a higher level. Such images are common in polar regions, where ice-sheets possess very low temperatures. Often towering castles and high mountain ranges appear on the horizon. Such a huge optical illusion is named a Fata Morgana. Morgana, in the legends of King Arthur, was a sorceress with the ability to invent castles in the sky to lure passing sailors and pirates to false landmasses. King Arthur’s seat lies at Tintagel castle in the county of Cornwall on a wild, jagged Atlantic coastline where hundreds of ships have been wrecked more through thick sea fog than fantasy. Whatever, Morgana’s magical conjuring has given this phenomenon her name.

A Fata Morgana can be explained by multiple mirages of warm and cold air sandwiched together, thus refracting light into different and distant images. Such distortions of reality were experienced by the Northwest Passage explorer, Captain John Ross, in his search for the northerly sea link between the Atlantic and Pacific Oceans. One August, 201 years ago, in the Lancaster Sound, he saw what he thought to be a huge mountain range that blocked his passage. This so called mountain range, he named the Croker Range. A few years later, a ship sailed right through this sound to the disbelief of Captain Ross!

It is through the curvature of the Earth that sunlight rays become bent and in doing so create false images to the naked eye. Last September, I travelled by hydrofoil through the Straits of Messina between Sicily and Calabria in southern Italy. Fata Morganas are often seen there on very hot days but, alas, I did not witness any elongated shapes in the sky despite the roasting temperatures.

Mirages at night

It was in 2002, on a small passenger ship in the Indian Ocean, en route from Kenya to Madagascar, that I observed such a phenomenon. At about 10pm, sitting with other passengers in the open air of the ship’s stern, we saw a frightening optical illusion of a huge ship bearing down on our small vessel. It looked approximately the size of an aircraft carrier! Had our captain cut across the bows of this massive ship? No, for within five minutes this apparition had evaporated into thin air. This mirage was occasioned by the combination of the full Moon’s rays and the ship’s deck lights, thus causing light refraction. There was a temperature inversion where the retained heat of the ocean’s surface air was greater than 10 metres or so above in colder air. A momentary frightening experience indeed.

Desert dust and sand

For most of us on Earth the current positions of the ever-meandering stratospheric Jet streams dictate the daily weather our atmosphere provides at ground level. In mid-February this year, the UK recorded its second highest February temperatures since official records were established in 1910. Remember that February is a winter month so far north of the Equator. Over 20 degrees Celsius was recorded for many places for five days or more. Some people sunbathed on beaches and even swam in sea. The meanderings of the Jet stream and the amplitude of its waves brought southerly air northwards, in abundance, from thousands of kilometres away from the Sahara Desert. Summer-like anticyclonic weather prevailed over much of Europe.

An inferior mirage spotted in the Mojave Desert. – Photo by Brocken Inaglory

At this particular time of the year, the Sahara Desert experiences very strong winds and sand storms which lift fine particles of sand skywards into the upper atmosphere and stratosphere to be carried onwards to other latitudes, passing over both land and sea. This ‘desert dust’ even drifts across the Atlantic Ocean to Amazonia in Brazil. There, researchers have calculated that 40 million tonnes of Saharan dust fall annually and provide 50 per cent of the rainforest’s needs in iron and phosphorus soil uptake. In crossing the Atlantic the dust falls out into the sea to provide vital minerals to plankton and food for fish and other sea creatures. Saharan dust literally helps plankton algae to bloom.

These minute particles of dust also absorb carbon dioxide, thus cooling the climate in some areas by reflecting sunlight back into space. For me, it is about time that I wash off Saharan dust from my car for fear of the corrosion of paintwork let alone to see through the windscreen where I am driving.

Such atmospheric events have to be seen to be believed. Whilst my meteorological knowledge does not extend far into the realms of atmospheric physics, I am sure that readers have experienced similar events as mine, in both Sabah and Sarawak and elsewhere in our world, to include the amazing colours of the skies as dawn breaks and at sunsets. The refractions of sunlight and moonlight rays above our heads do enliven our everyday lives wherever we may be on Earth.